Steganography, the art of hiding messages inside other messages, has until recently been the poor cousin of cryptography. Now, it is gaining new popularity with the current industry demands for digital watermarking and fingerprinting of audio and video.

Perhaps when you were a child, you used lemon juice to write text on paper, and then let the paper dry. Your writing would miraculously reappear on the apparently blank sheet of paper when you heated it.

Or perhaps when you were older, and were introduced to money, you noticed the image, or watermark, that would appear on bank notes when they were held up to the light. Both these types of situations are examples of steganography, the art of secret writing.

Steganography, from the Greek, means covered or secret writing, and is a long-practiced form of hiding information. Although related to cryptography, they are not the same. Steganography's intent is to hide the existence of the message, while cryptography scrambles a message so that it cannot be understood.

More precisely,``the goal of steganography is to hide messages inside other harmless messages in a way that does not allow any enemy to even detect that there is a second secret message present.'' Steganography includes a vast array of techniques for hiding messages in a variety of media. Among these methods are invisible inks, microdots, digital signatures, covert channels and spread-spectrum communications. Today, thanks to modern technology, steganography is used on text, images, sound, signals, and more.

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